A variety of diseases are modulated by bacterial and other toxins. Antitoxins to treat many such diseases exist, often as antibodies against the relevant toxin molecule. For example, antibodies against diphtheria are raised (classically, in horses) and administered as an antitoxin against diphtheria. However, even where such antitoxins are available, they may not provide optimal protection, and often have undesired side effects. Moreover, for some serious illnesses such as Bacillus anthracis infection, effective antitoxins are not available at all.
Anthrax is caused by the spore-forming, gram-positive bacterium Bacillus anthracis1. The disease is elicited when spores are inhaled, ingested or transmitted through open wounds in the skin. Inhalational anthrax is the deadliest form of the disease primarily because it is difficult to diagnose in a timely manner. Disease symptoms are initially nonspecific and systemic dissemination of anthrax toxin can occur prior to antibiotic treatment2. The deliberate release of B. anthracis spores in the U.S. in 2001 with the ensuing human fatalities and enormous cleanup costs has underscored the need for better detection, treatment and prevention of anthrax.
The toxic effects of anthrax are predominantly due to an AB-type toxin made up of a single, receptor-binding B subunit and two enzymatic A subunits3. The A subunits are edema factor (EF, 89 kD), an adenylate cyclase that raises intracellular cAMP levels4, and lethal factor (LF, 90 kD), a zinc protease that cleaves mitogen-activated protein kinase kinases5,6. The receptor binding B subunit is protective antigen (PA), which is initially synthesized as an 83 kD precursor. Upon receptor binding, PA83 is cleaved by furin into a 63 kD product, which forms heptamers that bind EF to form edema toxin (EdTx) and LF to form lethal toxin (LeTx)3. Two anthrax toxin receptors, widely distributed on human cells, have been identified: anthrax toxin receptor/tumor endothelial marker 8 (ANTXR1)7 and capillary morphogenesis gene 2 (ANTXR2)8. Although both receptors bind PA through a 200 amino acid extracellular von Willebrand factor A (VWA) domain, the VWA domain of ANTXR2 has a 1000-fold higher binding affinity for PA than the VWA domain of ANTXR1. In addition, ANTXR2 has been shown to mediate intoxication in vivo9-11. Recently, the LDL receptor-related protein LRP6 was shown to function as a co-receptor for anthrax toxin internalization12.
The potential use of anthrax and other diseases as weapons of bioterrorism has prompted increased efforts to develop better antitoxins and vaccines. In the case of anthrax, protective immunity to B. anthracis infection is conferred by antibodies against PA, which is the primary component of anthrax-vaccine adsorbed (AVA; Biothrax), the only currently licensed anthrax vaccine in the US. Although AVA is safe and effective, it is molecularly ill-defined, can cause adverse reactions and is administered in a lengthy immunization schedule (6 doses over 18 months)13. A second-generation vaccine based on recombinant PA adsorbed on aluminum hydroxide as adjuvant is currently in development. Preliminary data indicate that it is less potent than AVA and it is likely that several immunizations will be required to confer protection in humans14. Thus, the development of a well-characterized vaccine that induces rapid immunity after a single injection remains an important goal.
A general and widely adaptable vaccine platform to raise an immune response against anthrax and/or other serious diseases would be highly desirable. The present invention provides such a platform for use as either an antitoxin or a vaccine platform.